Film coefficients, heat transfer, data

Although Eq. 15.337 fits data for film mean thickness for nonturbulent films, the heat transfer coefficient is enhanced in the wavy region Chun and Seban [357] give the following expression taking account of this enhancement ... 

The wall heat transfer coefficient can be predicted by a model that is analogous to that outlined here for [72,73]. It should be stressed here that is intrinsically different from the global coefficients discussed in Sec. 11.5.a. Indeed, the latter are obtained when the experimental heat transfer data are analyzed on the basis of a one-dimensional model that does not consider radial gradients in the core of the bed. This comes down to localizing the resistance to heat transfer in radial direction completely in the film along the wall. 

Example Buckingham Pi Method—Heat-Transfer Film Coefficient It is desired to determine a complete set of dimensionless groups with which to correlate experimental data on the film coefficient of heat transfer between the walls of a straight conduit with circular cross section and a fluid flowing in that conduit. The variables and the dimensional constant believed to be involved and their dimensions in the engineering system are given below ... 

Najjar, Bell, and Maddox studied the influence of physical property data on calculated heat transfer film coefficients and concluded that accurate fluid property data is extremely important when calculating heat transfer coefficients using the relationships offered by Dittus-Boelter, Sieder-Tate, and Petukhov. Therefore, the designer must strive to arrive at good consistent physical/thermal property data for these calculations. 

Interpreting the plotted data from the authors tests indicates that heat transfer film coefficients at the scraped wall might he expected to range ... 

While it is possible to calculate the existing overall heat transfer coefficient from the operating data, it is not possible to calculate the individual film transfer coefficients. The individual film transfer coefficients can be combined in any number of ways to add up to an overall value of 285 W-m 2-K 1. However, the film transfer coefficients can be estimated from the correlations in Appendix C. Given that the tube-side correlations are much more reliable than the shell-side correlations, the best way to determine the individual coefficients is to calculate the coefficient for the tube-side and allocate the shell-side coefficient to add up to U = 285 W-m 2-K 1. Thus, to calculate the tube-side film transfer coefficient, KhT must first be determined. 

Summary of experimental data Film boiling correlations have been quite successfully developed with ordinary liquids. Since the thermal properties of metal vapors are not markedly different from those of ordinary liquids, it can be expected that the accepted correlations are applicable to liquid metals with a possible change of proportionality constants. In addition, film boiling data for liquid metals generally show considerably higher heat transfer coefficients than is predicted by the available theoretical correlations for hc. Radiant heat contribution obviously contributes to some of the difference (Fig. 2.40). There is a third mode of heat transfer that does not exist with ordinary liquids, namely, heat transport by the combined process of chemical dimerization and mass diffusion (Eq. 2-162). 

For non-Newtonian liquids and suspensions, an apparent viscosity is determined using correlations which include power input and the Reynolds number. Scale-up comparisons based on heat generation data only were determined by comparison of results from RC1 experiments and from a 675-liter reactor [208]. In the experiments, a Bingham plastic fluid was used to determine the film heat transfer coefficient. This presents a worst case because of the low thermal conductivity of the Bingham plastic. Calculated inside film heat transfer coefficients determined in the RC1 tests were about 60% lower than the values determined in the pilot plant reactor, even though substantial effort was made to obtain both geometric and kinematic similarity in the pilot reactor. 

If data are available we can calculate whether film resistance to heat transfer is important by the estimate of Eq. 36., and whether film resistance to mass transport is important by comparing the observed first-order rate constant based on the volume of particle with the mass transfer coefficient for that type of flow. 

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